A photon scanning tunneling microscope for detecting the evanescent light localized in small-sized areas on an sample surface smaller than the wavelength of light has been known as an ultra-high resolution optical microscope having resolution beyond the diffraction limit of a conventional optical microscope.
For example, if the sample surface of a sample 50 is irradiated from its back surface under total internal reflection conditions, an evanescent field is generated on the sample surface depending on the surface shape, as shown in FIG. 1.
With the photon scanning tunneling microscope, the intensity of the evanescent field is measured with an optical probe 52 having a sharpened core 51 with an aperture smaller than the wavelength of the evanescent light, as shown for example in FIG. 2. That is, if the apex of the sharpened core 51 is approached to less than a distance on the order of the evanescent optical wavelength, the evanescent light is scattered by the apex of the sharpened core 51 so as to be propagated in the core. The light propagated in the core may be detected on the opposite side to the detection end for measuring the intensity of the evanescent field. The shape of the sample surface can be measured by finding the distribution of intensity of the evanescent field by scanning the sample surface. With this photon scanning tunneling microscope, the resolution is determined by the shape of the apex of the sharpened core 51, so that, by employing an optical probe 52 having the apex size of the sharpened core 51 shorter than the optical wavelength as described above, the resolution beyond the diffraction limit of a conventional optical microscope can be achieved.
The resolution of the photon scanning tunneling microscope is determined by the effective aperture diameter of the optical probe. On the other hand, the intensity of the evanescent field decreases exponentially with the distance from the sample surface. Consequently, the aperture diameter of the optical probe can be decreased by simply decreasing the apex size of its tip. Thus, for improving the resolution of the photon scanning tunneling microscope, it is critical to sharpen the tip of the optical probe.
Consequently, various methods have been tested for preparing an optical probe having a sharpened tip. An optical fiber is produced by a method consisting in sharpening an end of an optical fiber made up of a core doped with germanium dioxide and a clad.
However, since a clad diameter D (on the order of 90 .mu.m) of a conventional optical fiber 52 of this type is significantly larger than the length L of a sharpened core 51 (on the order of 2 to 6 .mu.m), a peripheral portion 52 of the clad tends to impinge on a sample 50 to damage the sample and/or the probe. Also, since the output of the evanescent light is extremely small, it is necessary with the optical fiber detecting the evanescent light (power) in order to avoid the effect by the scattered light and in order to raise the detection efficiency.
On the other hand, an optical coupling element optically coupling the light propagated in an optical fiber with an optical waveguide includes an optical fiber 60, a lens 61 and a optical waveguide 62, as shown in FIG. 3. With this optical coupling element, the light from a core 63 of the optical fiber 60 is collected using a lens 61 and caused to be incident on the optical waveguide 62.
Another optical coupling element has a semispherical protrusion 64 on the apex of a core 63 on one end of the optical fiber 60, as shown for example in FIG. 4. This optical coupling element is so constructed that the light propagated in the core 63 is collected by the protrusion 64 and caused to fall on the optical waveguide 62.